Evolution of calls and auditory tuning in the Physalaemus pustulosus species group

In species within the Physalaemus pustulosus species group, male frogs produce a whine-like advertisement call consisting of a frequency sweep typically descending from 1,000 to 400 Hz (depending on the species). One species, Physalaemus pustulosus, the túngara frog, has evolved a second call syllable, the chuck, which males place after their whine. Most energy in the chuck is above 1,500 Hz and peaks at 2,400 Hz. We investigated whether the evolution of this new call component in P. pustulosus coincided with evolution of auditory tuning. We used multiunit electrophysiological recordings of auditory-evoked activity in the midbrain to characterize auditory tuning in Physalaemus pustulosus, four other Physalaemus species within the P. pustulosus clade, and three additional, closely related Physalaemus species as outgroups. All eight species had similar sensitivity profiles, with a broad area of enhanced sensitivity from 100 to 1,100 Hz, which we presume represents amphibian papilla (AP) tuning, and a second, narrower area of enhanced sensitivity centered above 2,100 Hz, which we presume represents basilar papilla (BP) tuning. For all species, the whine stimulates the AP. The P. pustulosus chuck stimulates the BP. The frequency with greatest AP sensitivity differed significantly among species. Although in all cases the AP peak lay within the frequency sweep of the whine, phylogenetically corrected correlations revealed no significant relationships between AP tuning and any spectral feature of the whine. BP tuning was similar among all species, with mean BP best excitatory frequencies (BEFs) around 2,100-2,200 Hz, with the exception of P. pustulatus, with a mean BP BEF of 2,549 Hz. Physalaemus pustulosus, the only investigated species that produces a call component stimulating the BP, had a BP BEF that was not significantly different from any of the species within its clade except P. pustulatus, or from any of the outgroup species. A phylogenetic reconstruction of ancestral BP tuning confirms that the only point of evolutionary change in BP tuning is in the line of descent leading to P. pustulatus, not in the line leading to P. pustulosus despite this being the species using the BP for communication. The results indicate that BP tuning around 2,200 Hz is a conserved trait in the Physalaemus pustulosus species group and that no evolution of BP tuning accompanied the subsequent evolution of the call component (the chuck) that stimulates it. This supports the sensory exploitation idea, which posits that signals evolve to match preexisting features of receiver systems.